Apparatus and methods of operation of passive led lighting equipment

ABSTRACT

This invention is concerned with the control and design of a LED lighting system that does not need electrolytic capacitors in the entire system and can generate light output with reduced luminous flux fluctuation. The proposal is particularly suitable, but not restricted to, off-line applications in which the lighting system is powered by the ac mains. By eliminating electrolytic capacitors which have a limited lifetime of typically 15000 hours, the proposed system can be developed with passive and robust electrical components such as inductor and diode circuits, and it features long lifetime, low maintenance cost, robustness against extreme temperature variations and good power factor. No extra electronic control board is needed for the proposed passive circuits, which can become dimmable systems if the ac input voltage can be adjusted by external means.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/474,001, filed May 28, 2009, which is a continuation-in-partof U.S. patent application Ser. No. 12/429,792, filed Apr. 24, 2009,each of which is hereby incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

This invention relates to apparatus and methods for the operation ofpassive light emitting diode (LED) lighting equipment, and in particularto such apparatus and methods as may avoid the need to use electrolyticcapacitors.

BACKGROUND OF THE INVENTION

LED technology has been promoted as a promising lighting technology toreplace energy-inefficient incandescent lamps and mercury-based linearand compact fluorescent lamps. It is often claimed by LED manufacturersthat the LED devices have a long lifetime that could be higher than 5years. However, the electrolytic capacitors used in the power circuitand the electronic controls for LED systems have a limited lifetime,typically 15000 hours (or 1.7 years) at an operating temperature of 105°C. The lifetime of an electrolytic capacitor is highly sensitive to theoperating temperature. The lifetime is doubled if the operatingtemperature is decreased by 10° C. and halved if increased by 10° C.Therefore, the short lifetime of electronic control circuits (sometimesknown as ballasts) for LEDs remains one major bottleneck in theutilization of LED technology [Chung, H. S.-H.; Ho, N.-M.; Yan, W.; Tam,P. W.; Hui, S. Y.; “Comparison of Dimmable Electromagnetic andElectronic Ballast Systems—An Assessment on Energy Efficiency andLifetime”, IEEE Transactions on Industrial Electronics, Volume 54, Issue6, December 2007 Page(s): 3145-3154; Hui S. Y. R. and Yan W.,“Re-examination on Energy Saving & Environmental Issues in LightingApplications”, Proceedings of the 11^(th) International Symposium onScience 7 Technology of Light Sources, May 2007, Shanghai, China(Invited Landmark Presentation), pp. 373-374].

In general, electrolytic capacitors are used in power inverter circuitsand electronic control circuits for lighting systems because theyprovide the necessary large capacitance of the order of hundreds andeven thousands of micro-Farads, while other more long-lasting capacitorssuch as ceramic, polypropylene and metalized plastic film capacitorshave relatively less capacitance of several tens of micro-Farads orless. The large capacitance of electrolytic capacitors is usually neededto provide a stable dc link voltage for the ballast circuit to providestable power (with reduced power variation) for the load; a stable dcpower supply in the electronic control for the power inverter circuit.

FIG. 1 shows the schematic of a typical off-line lighting system. Anoff-line system here means a system that can be powered by the ac mains.The power conversion circuit can adopt a two-stage approach in which anAC-DC power stage with power factor correction is used as the firstpower stage, which is followed by a second dc-dc power conversion stagefor controlling the current for LED load. An alternative to thetwo-stage approach is to employ a single-stage approach which combinesthe two power stages into one and such a technique has been reported inmany off-line power supply designs [Reis, F. S. D.; Lima, J. C.;Tonkoski, R., Jr.; Canalli, V. M.; Ramos, F. M.; Santos, A.; Toss, M.;Sarmanho, U.; Edar, F.; Lorenzoni, L.; “Single stage ballast for highpressure sodium lamps”, IECON 2004. 30th Annual Conference of IEEEIndustrial Electronics Society, 2004. Volume 3, 2-6 Nov. 2004 Page(s):2888-2893 Vol. 3; Jinrong Qian; Lee, F. C.; “A high efficient singlestage single switch high power factor AC/DC converter with universalinput”, Twelfth Annual Applied Power Electronics Conference andExposition, 1997. APEC '97 Conference Proceedings 1997, Volume 1, 23-27Feb. 1997 Page(s): 281-287; Qiao, C.; Smedley, K. M.; “A topology surveyof single-stage power factor corrector with a boost typeinput-current-shaper”, IEEE Transactions on Power Electronics, Volume16, Issue 3, May 2001 Page(s): 360-368; Tse, C. K.; Chow, M. H. L.;“Single stage high power factor converter using the Sheppard-Taylortopology”, 27th Annual IEEE Power Electronics Specialists Conference,1996. PESC '96 Record., Volume 2, 23-27 Jun. 1996 Page(s): 1191-1197vol. 2]. In both approaches, electrolytic capacitors are used to providethe energy storage and buffer so that the difference between the inputpower and the output power consumed by the load can be stored ordelivered by the capacitors.

Regardless of whether a single-stage or a two-stage approach is used, alarge capacitance (requiring the use of electrolytic capacitors) isneeded as energy-storage to cater for the difference between the inputpower from the ac mains and the almost constant power of the LED load.The input power of an off-line lighting system is typically aperiodically pulsating function as shown in FIG. 1. For example, ifpower factor is close to one, the input voltage and current are in phaseand thus the input power follows a pulsating waveform (similar to arectified sinusoidal waveform). If the lighting load is of constantpower, then the capacitors are needed to absorb or deliver thedifference in power between the ac mains and the lighting load as shownin FIG. 1.

An electronic ballast circuit without the use of electrolytic capacitorshas been proposed. But the requirement for active power switches in suchproposal means that an electronic control board that provides theswitching signals for the active power switches is needed and thiselectronic control board needs a power supply that requires the use ofelectrolytic capacitors. In general, electrolytic capacitors are neededin a dc power supply for providing the hold-up time (i.e. to keep the dcvoltage for a short period of time when the input power source fails.)Power electronic circuits that use active switches usually need a dcpower supply for the gate drive circuits that provide switching signalsfor the active electronic switches. Therefore, it would be useful if apassive electronic ballast circuit can be developed for providing astable current source for the LED load. A passive ballast circuitwithout active switches, electronic control board and electrolyticcapacitors would be a highly robust and reliable solution that enhancesthe lifetime of the entire LED system. The remaining challenge is todetermine how to provide a stable current source for the LED load basedon a totally passive circuit.

SUMMARY OF THE INVENTION

According to the present invention there is provided an LED lightingsystem comprising in sequence: (a) a rectification circuit forrectifying an AC input power and generating a rectified DC power, (b) afirst circuit for reducing the voltage ripple of said rectified DCpower, (c) a second circuit for generating a current source, and (d) atleast one LED receiving said current source as an input.

Preferably the voltage ripple reducing circuit is a valley-fill circuitlocated between the rectification circuit and the inductor. Thevalley-fill circuit may include a voltage-doubler.

Preferably, the valley-fill circuit includes a first capacitor and asecond capacitor. The capacitances of the first and second capacitorsmay be the same, or the first and second capacitors may have differentcapacitances such that the voltage ripple of said rectified DC power isfurther reduced.

Preferably the second circuit comprises an inductor. The second circuitmay further function as a current ripple reduction circuit. Such acurrent ripple reduction circuit may comprise a coupled inductor with acapacitor.

Preferably means are also provided for reducing the sensitivity of theLED power to fluctuations in the AC input supply. This may be achieved,for example, by placing an inductor in series between the AC inputsupply and the diode rectification circuit. A capacitor may also beprovided in parallel between this input inductor and the dioderectification circuit.

This input inductor may be a variable inductor that is controllable suchthat the at least one LED is dimmable. The use of a variable inductormay solely be for providing a dimming function, or may be for reducingthe sensitivity of the LED power to fluctuations in the AC input supplyin combination with providing a dimming function.

Such use of an input inductor may also be useful independently ofproviding reduction of voltage/current ripple and therefore according toanother aspect of the invention there is also provided an LED lightingsystem comprising: an AC input power source, a rectification circuit forrectifying an AC input power and generating a rectified DC power, and aninductor provided in series between the AC input power source and therectification circuit. Again, a capacitor may be provided in parallelbetween the inductor and the diode rectification circuit. Also, theinput inductor may be a variable conductor that is controllable so thatthe LED lighting system is dimmable.

In preferred embodiments of the invention the power supplied to the atleast one LED is permitted to vary, and the operating and/or designparameters of the at least one LED are chosen such that the variation inluminous flux resulting from the variation in power is not observable tothe human eye.

Viewed from another broad aspect the present invention provides a methodof operating a LED lighting system comprising the steps of: (a)rectifying an AC input voltage to generate a rectified DC power, (b)reducing the voltage ripple of the rectified DC power, (c) generating acurrent source from the voltage ripple reduced rectified DC power, and(d) providing the current source as an input to at least one LED,wherein the power supplied to the at least one LED is permitted to vary,and wherein the operating and/or design parameters of the at least oneLED are chosen such that the variation in luminous flux resulting fromthe variation in power is not observable to the human eye.

Preferably a thermal characteristic of the at least one LED may bechosen such that the variation in luminous flux resulting from thevariation in power is not observable to the human eye. Such a thermalcharacteristic may comprises the design of the heatsink and/or theprovision of forced cooling or natural cooling.

Preferably a valley-fill circuit is used to reduce the voltage ripple ofthe rectified DC power. The valley-fill circuit may include avoltage-doubler.

Preferably, the valley-fill circuit is provided with a first capacitorand a second capacitor. The capacitances of the first and secondcapacitors may be the same, or the first capacitor may be selected witha different capacitance to the second capacitor such that thevalley-fill circuit is used to further reduce the voltage ripple of therectified DC power.

In preferred embodiments of the invention the method further comprisesthe step of reducing the current ripple of said current source. Such acircuit may comprise a coupled inductor with a capacitor used to reducethe current ripple.

Preferably the sensitivity of the LED power to fluctuations in the ACinput supply voltage is also controlled.

Preferably, the AC input voltage can be varied so that the LED lightingsystem is dimmable.

The valley-fill circuit described above can be used more generally togenerate a DC output voltage for broader variety of applications.Therefore, in another broad aspect of the present invention, there isprovided a valley-fill circuit for generating a DC output voltage, thecircuit including a first capacitor and a second capacitor, wherein thefirst and second capacitors have different capacitances such that thevoltage ripple of the DC output voltage is reduced.

A further broad aspect of the present invention provides a method ofgenerating a DC output by using a valley-fill circuit including a firstcapacitor and a second capacitor, wherein the first and secondcapacitors have different capacitances such that a DC output voltagewith reduced voltage ripple is generated.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention will now be described by way ofexample and with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic and power profiles of a typical off-line LEDlighting system according to the prior art;

FIG. 2 shows a schematic and “modified” power profiles of an off-lineLED lighting system according to an embodiment of the invention;

FIGS. 3( a)-(c) show the variation of LED power and luminous flux in anembodiment of the present invention;

FIGS. 4( a), (b) and (c) show (a) a schematic diagram of a passiveoff-line circuit design for an LED system using an inductor for currentripple reduction, and (b) and (c) using a coupled inductor for currentripple reduction;

FIG. 5 shows a schematic of an example of one possible hardwareimplementation of the proposed passive circuit for an off-line LEDsystem using a standard valley-fill circuit;

FIG. 6 shows a model used for simulation of the circuit in FIG. 5;

FIG. 7 shows an example of a proposed passive circuit with a standardvalley-fill circuit for multiple loads;

FIG. 8 shows an example of a proposed passive circuit using avalley-fill circuit with a voltage doubler for multiple loads;

FIG. 9 shows an LED system according to an embodiment of the inventionunder a simulation evaluation (L=1H);

FIGS. 10( a) and (b) show (a) simulated input voltage and current of thesystem of FIG. 9, and (b) simulated input power of the system of FIG. 9;

FIGS. 11( a)-(d) show (a) simulated voltage and current of the LEDmodule for the circuit of FIG. 9, (b) simulated total power for the LEDmodule and for individual LEDs in the module for the system in FIG. 9,(c) and (d) two examples of the relationship between a variation of LEDpower and luminous flux fluctuation for a LED system using 3 W LEDdevices;

FIG. 12 shows an LED system according to an embodiment of the inventionunder a simulation evaluation (L=2H);

FIGS. 13( a)-(d) show (a) simulated input voltage and current of thesystem of FIG. 12, (b) simulated input power of the system of FIG. 12,(c) and (d) two examples of the relationship between a variation of LEDpower and luminous flux fluctuation for a LED system using 3 W LEDdevices;

FIG. 14 shows an embodiment of a LED system with “coupled inductor” ofL=2H under simulation evaluation (L=2H);

FIGS. 15( a)-(d) show (a) simulated input voltage and current of thesystem of FIG. 14, (b) simulated input power of the system of FIG. 14,(c) and (d) two examples of the relationship between a variation of LEDpower and luminous flux fluctuation for a LED system using 3 W LEDdevices;

FIG. 16 shows a diode-clamp that may be added to each LED string inembodiments of the invention;

FIGS. 17( a) and (b) illustrate the use of the valley-fill circuit inreducing the voltage ripple;

FIG. 18 shows a circuit according to a further embodiment of theinvention;

FIGS. 19( a)-(d) show idealized waveforms in the circuit of FIG. 18;

FIG. 20 shows a simplified equivalent circuit of FIG. 18;

FIG. 21 shows a vectorial relationship in the equivalent circuit of FIG.20;

FIG. 22 shows a circuit according to a still further embodiment of theinvention;

FIG. 23 shows a circuit according to a still further embodiment of theinvention;

FIG. 24 shows a circuit according to an embodiment of the invention inwhich the circuit includes a variable inductor L_(s);

FIG. 25 shows the variable inductor L_(s) of FIG. 24 based on tappingcontrol;

FIG. 26 shows the variable inductor L_(s) of FIG. 24 based on coresaturation;

FIG. 27 is a graph showing the output voltage of a diode bridge onlycircuit;

FIG. 28( a) is a graph showing the output voltage of the circuit of FIG.18 in which C1=C2=220 μF;

FIG. 28( b) is a graph showing the output voltage of the circuit of FIG.18 in which C1=C2=22 μF;

FIG. 29 show capacitors C1 and C2 connected in series; and

FIG. 30 is a graph showing the output voltage of the circuit of FIG. 18in which C1=6600 μF and C2=330 μF.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One important aspect of this invention at least in its preferred formsis to provide a way to reduce the size of the capacitors that is neededso that capacitors other than the electrolytic type can be used. Withelectrolytic capacitors eliminated in the lighting system, the wholesystem can be more reliable and last longer.

FIG. 2 is a modified version of FIG. 1 and is used to illustrate thisaspect of the invention. If the LED load power is allowed to fluctuateto some extent, the amount of energy buffer required in theenergy-storage element of the system becomes less and therefore the sizeof the capacitance can be reduced to a level that other non-electrolyticcapacitors can be used to replace the electrolytic capacitor.Furthermore as the circuit contains only passive components rather thanactive components complicated control circuitry (which may also requireelectrolytic capacitors) can be avoided.

In addition to the elimination of electrolytic capacitors, the design isalso concerned with the input power factor because there is aninternational standard IEC-61000 governing the input power factor.Passive power correction circuits such as valley-fill circuits and theirvariants [K. Kit Sum, “Improved Valley-Fill Passive Current Shaper”,Power System World 1997, p. 1-8; Lam, J.; Praveen, K.; “A New PassiveValley Fill Dimming Electronic Ballast with Extended Line CurrentConduction Angle”, INTELEC '06. 28th Annual InternationalTelecommunications Energy Conference, 2006. 10-14 Sep. 2006 Page(s):1-7] can be used in the passive ballast circuit in embodiments of thisinvention.

Valley-fill circuits allow the input current to be smoothed so that thecurrent distortion factor and thus the input power factor can beimproved. The choice of the capacitors used in the valley-fill circuitcan be made so that non-electrolytic capacitors can be used. Unlikeprevious applications, the valley-fill circuit is used in embodiments ofthis invention to reduce the output voltage ripple which in turn willreduce the current ripple in the later power stage. This aspect of thevalley-fill circuit application has not been reported previously becausein the prior art valley-fill circuits were primarily used for voltagesource applications and were used as a means for input power factorcorrection with their outputs are nominally connected directly toanother power converter or a load. For example, in the NationalSemiconductor Note: LM3445 Triac Dimmable Offline LED Driver March 2009,the two capacitors C7 and C9 in the valley-fill circuit are electrolyticcapacitors and the valley-fill circuit provides a “voltage source” to abuck converter which in turn controls the power of the LED load. Suchexample of valley-fill circuit application highlights the traditionaluse of “electrolytic capacitor” in absorbing large power variation andthe voltage source nature of prior art.

In contrast in embodiments of the present invention valley-fill circuitsare used to reduce the output voltage ripple. As shown in FIG. 17( a),the output voltage of the diode rectifier has high voltage ripple.However, the output voltage of the valley-fill circuit is significantlyreduced as shown in FIG. 17( b). In embodiments of this invention, thevalley-fill circuit is not connected directly to the load or anotherpower converter as in prior art, but is connected directly to aninductor or a coupled-inductor based current ripple cancellation circuitfor providing a smooth current to the LED load.

In embodiments of the invention an inductor (FIG. 4( a)) or a coupledinductor with ripple cancellation (FIG. 4( b)) may be used to limit theoutput current ripple and hence the power variation for the LED load.

FIG. 4( a) and FIG. 4( b) show schematic diagrams of passive circuitsaccording to embodiments of the invention that can provide highreliability, long lifetime and low cost. Each system consists of a dioderectifier, a valley-fill circuit for improving the input power factor,an inductor for turning the voltage source into a current source withreduced current ripple (FIG. 4( a)) and the LED load. An alternativeembodiment as shown in FIG. 4( b) is to replace the inductor in FIG. 4(a) with a coupled inductor and a capacitor so that these components forma coupled inductor with current ripple cancellation function. It will beshown that such current ripple cancellation which is commonly used inhigh-frequency (greater than 20 kHz) switching power supplies can alsobe effective in low-frequency operation. The LED load could be an LEDarray or multiple arrays in modular forms. Various valley-fill circuitsor their improved versions can be used to improve the input powerfactor. In embodiments of this invention, non-electrolytic capacitorscan be used in the valley-fill circuit and current-ripple cancellationcircuit. Either a standard valley-fill circuit, a valley-fill circuitwith voltage doubler or any variant of the valley-fill circuit can beused in this invention.

Considering firstly FIG. 4( a), let the output voltage of thevalley-fill circuit be V_(out) and the overall voltage of the LED module(with LED devices connected in series) be V_(LED). The inductance of theinductor can be designed to limit the current through the LED modulebecause the current ripple ΔI_(LED) can be expressed as:

$\begin{matrix}{{\Delta \; I_{LED}} = \frac{\left( {V_{out} - V_{LED}} \right)\Delta \; t}{L}} & \mspace{11mu}\end{matrix}$

where Δt is the time period during the current change.

From the above equation, it can be seen that the size of the inductor Lcan be used to reduce the current ripple, which in turn can limit thechange of total LED power because

ΔP_(LED)=V_(LED)ΔI_(LED)

An alternative shown in FIG. 4( b) is to use a coupled inductor withcurrent ripple cancellation as described in the art [Hamill, D. C.;Krein, P. T.; “A ‘zero’ ripple technique applicable to any DCconverter”, 30th Annual IEEE Power Electronics Specialists Conference,1999. PESC 99. Volume 2, 27 Jun.-1 Jul. 1999 Page(s): 1165-1171;Schutten, M. J.; Steigerwald, R. L.; Sabate, J. A.; “Ripple currentcancellation circuit” Eighteenth Annual IEEE Applied Power ElectronicsConference and Exposition, 2003. APEC '03. Volume 1, 9-13 Feb. 2003Page(s): 464-470; Cheng, D. K. W.; Liu, X. C.; Lee, Y. S.; “A newimproved boost converter with ripple free input current using coupledinductors”, Seventh International Conference on Power Electronics andVariable Speed Drives, 1998. (Conf. Publ. No. 456) 21-23 Sep. 1998Page(s): 592-599]. The primary winding of the coupled inductor is usedas the dc inductor just as in the embodiment of FIG. 4( a). Thesecondary winding is coupled to the primary winding and provides the accurrent to reduce the ripple in the load. When the primary current inthe first inductor is increasing into the dotted terminal of the primarywinding (i.e. changing positively), ac flux caused by the increasingprimary current is coupled to the secondary ac winding. The transformeraction causes a current to flow out of the dotted terminal of thesecondary winding into a capacitor in order to cancel the ac flux. Thus,the overall current ripple in the output of the coupled inductor(including both primary and secondary windings) and the load is reduced.Similarly, when the primary current flowing into the dotted terminal ofthe primary winding is decreasing (i.e. changing negatively), the acflux coupled to the secondary winding will cause a current to flow intothe dotted terminal of the secondary winding and hence reduce theoverall current ripple of the couple inductor. The effect of the coupledinductor on reducing the current ripple is illustrated in FIG. 4 (c).

In embodiments of the present invention there will be fluctuation of theLED load power, but it is possible to obtain luminous output from theLED system with minimum luminous flux fluctuation even though the LEDload power will fluctuate. This can be seen by considering therelationship between the luminous flux φ_(v) and LED power P_(d) asshown in FIGS. 3( a)-(c). Let us label the maximum power and minimumpower of the LED load as Pmax and Pmin, respectively in FIG. 3( a). Ithas been shown that the relationship of the luminous flux and the powerof a LED system follows an asymmetric parabolic curve as shown in FIG.3( b) [Hui S. Y. R. and Qin Y. X., “General photo-electro-thermal theoryfor light-emitting diodes (LED) systems”, IEEE Applied Power ElectronicsConference, February 2009, Washington D.C., USA, paper 16.2; U.S. Ser.No. 12/370,101 the contents of which are incorporated herein byreference]. If the LED system is designed such that Pmax and Pminenclose the peak region of the luminous flux—LED power curve where theslope of the curve is minimum as shown in FIG. 3( b), a significantvariation of LED power (ΔP_(LED)) will only lead to a relatively smallvariation in the luminous flux (Δφ_(v)). An alternative is to design theLED thermal design so that P_(max) and P_(min) fall within a region ofthe luminous flux—LED power curve where the slope of the curve isrelatively small (i.e. near the peak value) as shown in FIG. 3( c).

In this way, the control circuit can use non-electrolytic capacitorswithout causing a large variation in the light output of the LED system.This concept can be implemented in existing electronic ballasts byreplacing the electrolytic capacitors with other capacitors of lowervalues and re-designing the LED system so that the LED power variationfalls within the peak luminous flux region in the luminous flux—LEDpower curve.

Another important aspect of the present invention involves the use ofnovel passive power circuits that can achieve the advantages proposedabove without using active electronic switches. Without using activeelectronics switches, the proposed circuits do not need an electroniccontrol circuit for the switches and can be much more reliable,long-lasting and have lower costs than their active electroniccounterparts.

FIG. 5 shows a circuit diagram based on a standard valley-fill circuit.In the actual simulation as shown in FIG. 6, a small number of LEDdevices are represented by individual diodes and a large number of theLED devices are represented by an equivalent resistor that has the samevoltage drop and consumes the same power of that group of LED deviceswhen the rated current flow through these series connected devices. Avalley-fill circuit with a voltage doubler as shown in FIG. 7 can alsobe used if desired. If multiple LED modules are used as shown in FIG. 8,current-balancing devices can be added to ensure that each LED arraymodule shares the same current.

In order to illustrate this aspect of the present invention, the passivecircuit of FIG. 9 is used to drive a series of 3 W LEDs. In thesimulation, three diodes are used while the rest of the diodes arerepresented as an equivalent resistor as explained previously. FIG. 10(a) shows the simulated input voltage and current of the entire system.It can be seen that the input current waveform is not a sharp pulse (aswould be expected from a diode bridge with an output capacitor) and thepower factor has therefore been improved. FIG. 10( b) shows the inputpower of the system. FIG. 11( a) shows the simulated voltage and currentof the LED module. The inductor is designed so that the LED ratedcurrent of 1 A (for the 3 W LED devices) is not exceeded in thisexample. Despite the pulsating input power, the reduction of the voltagefluctuation due to the use of the valley-fill circuit and the filteringeffect of the inductor have smoothed the load current considerably. FIG.11( b) shows the total LED power and individual LED power. It can beseen that the power variation is within 1.2 W to 3 W (i.e. 60%) in thisexample. This simulation study confirms that a passive circuit withoutelectrolytic capacitors and active switches can be designed to provide acurrent source with controlled current ripple for a LED system withinput power factor correction.

This per-unit result of LED power in FIG. 11 can be interpreted withtypical LED systems with different thermal designs. For example, it hasbeen shown that the luminous flux—LED power curves depend on the thermalresistance of the heatsinks. FIG. 11( c) and FIG. 11( d) show typicalcurves for LED systems using two different heatsinks for eight 3 W LEDs.The heatsink used for FIG. 11( c) is smaller than that for FIG. 11( d).For the example in FIG. 11( c), a 60% variation from 1.2 W to 3 W foreach device will lead to about 24% of light variation. For the exampleof FIG. 11( d), a 60% variation of LED power leads to 30% of lightvariation.

However, it is important to note that the choice of inductance of theinductor can control the current ripple and therefore the LED powervariation. If the inductance L is increased from 1H to 2H (FIG. 12), thesimulated LED voltage and current waveforms are plotted in FIG. 13( a).The corresponding total LED power and individual LED power are includedin FIG. 13( b).

It can be seen that, with L increased to 2H, the power variation (from1.6 W to 2.5 W) is 36%. If the same power variation is applied to thetwo examples in reference Hui et al [Hui S. Y. R. and Qin Y. X.,“General photo-electro-thermal theory for light-emitting diodes (LED)systems”, IEEE Applied Power Electronics Conference, February 2009,Washington D.C., USA, paper 16.2], FIG. 13( c) and FIG. 13( d) show thatthe variation in the luminous flux is approximately 7% and 12%,respectively. It is envisaged that human eyes are not sensitive to suchsmall changes of luminous flux variation.

It can be seen that a large inductance can reduce the current ripple andLED power variation. The choice of L depends also on the core loss andcopper loss in the inductor. The overall design therefore relies on thethermal design as explained in Hui et al and the choice of L so that theoperating range can be restricted to the region of the luminous flux—LEDpower curve where the slope of the curve is small.

An effective method to further reduce the current ripple and thus LEDpower variation and light variation is to replace the inductor in FIG. 9and FIG. 12 with a current-ripple cancellation means in the form of acoupled inductor and a capacitor as shown in FIG. 14. FIG. 15( a) andFIG. 15( b) show the electrical measurements of the system. It can beseen the variations in the LED current ripple and power have beengreatly reduced. The power variation is only within 0.2 W (from 1.9 W to2.1 W). This 9% power variation will lead to less than 4% of lightvariation in the two examples as shown in FIG. 15( c) and FIG. 15( d).

It should also be noted that it may be desirable to provide adiode-capacitor clamp that can be added to each LED string to provide acurrent path for the inductor current in case some of the LED devicesfail. An example of such a possibility is shown in FIG. 16.

From the above it will be seen that in preferred embodiments of thepresent invention there is proposed the use of a passive powercorrection circuit such as the valley-fill circuit to reduce the voltageripple feeding the inductor (or coupled inductor with a capacitor in theform of current ripple cancellation circuit) and the LED modules inorder to (i) reduce the current ripple and thus the power variation inthe LEDs and (ii) to improve the input power factor. The allowance ofsome current and power variation in the LEDs within the region of theluminous flux—LED power curve where the slope of the curve is small willlead to only a small variation of the luminous flux from the LED system.The use of the inductance of the inductor or coupled inductor in theform of a current ripple cancellation circuit to further limit the powervariation of the LED system.

By using a suitable thermal design the power variation range of the LEDload can be designed to fall within the region of the luminous flux—LEDpower curve where the slope is small and the luminous flux is maximum ornear maximum.

As a consequence of the requirement of only small capacitance in theproposed system, electrolytic capacitors can be eliminated from thisdesign. Since the entire circuit consists of passive and robustcomponents (such as power diodes, non-electrolytic capacitors andinductors) only and does not need extra control electronics, it featureslow-cost, high robustness and reliability.

One possible issue, however, is that the abovedescribed circuits assumea reasonably constant input voltage which may not necessarily be true.In countries where the AC mains supply is unreliable or in any othersituation where there may be AC mains voltage fluctuation for whateverreason, there could be a significant variation in the LED power for agiven nominal AC input voltage. In preferred embodiments of theinvention therefore it may be preferable to provide a means forcontrolling the power sensitivity of the load against AC voltagefluctuation.

FIG. 18 shows one example of a circuit provided with means forcontrolling the power sensitivity of the load against AC voltagefluctuation. In this example a passive ballast for an LED system isshown provided with a diode rectifier, a valley-fill circuit forreducing the voltage ripple of the rectified DC power, and a filterinductor L for generating a current source provided to the LED load. Itwill be understood that as described above the inductor L could insteadby replaced by a current ripple reduction circuit comprising a coupledinductor with a capacitor. In this circuit an input inductor L_(s) isprovided in series between the AC supply V_(S) and the diode rectifierwhich as will be explained below provides the necessary powersensitivity control.

FIGS. 19( a)-(d) show the idealized waveforms of the proposed AC-DCcurrent source circuit for LED loads. In particular: FIG. 19( a) showsidealized waveforms of input AC mains voltage and current (with a phaseshift (φ) between V_(s) and I_(s)); FIG. 19( b) shows idealizedwaveforms of input voltage V₂ and current I_(s) of the diode rectifier(with V₂ and I_(s) in phase); FIG. 19( c) shows idealized waveforms ofoutput voltage V₃ and current I_(o) of the valley-fill circuit (with V₃a rectified version of V₂); and FIG. 19( d) shows idealized waveforms ofvoltage across LED load (V_(o)), output load current (I_(o)) and theoutput load power (P_(o)).

An analysis of this circuit can start from the load side by consideringthe equivalent circuit as shown in FIG. 20 where the inductor windingresistance is shown as R and the total LED load voltage drop V₀ isconsidered to be constant.

From FIG. 20, the average output current Ī_(o) can be expressed as:

$\begin{matrix}{{\overset{\_}{I}}_{o} = \frac{{\overset{\_}{V}}_{3} - V_{o}}{R}} & (1)\end{matrix}$

where V ₃ is the average voltage of V₃.

From the waveform of V₃ in FIG. 19( c),

$\begin{matrix}{{\overset{\_}{V}}_{3} = {\frac{3}{4}V_{d\; c}}} & (2) \\{V_{d\; c} = {{\frac{4}{3}{\overset{\_}{V}}_{3}} = {\frac{4}{3}\left( {V_{o} + {{\overset{\_}{I}}_{o}R}} \right)}}} & (3)\end{matrix}$

It should be noted that the total voltage drop of the LED load isapproximated as a constant V_(o). Therefore, V_(dc) does not changesignificantly if Ī_(o) does not change significantly. In general, V_(o)is much bigger than Ī_(o)R. Thus V_(dc) is close to 1.33 V_(o). The nextissue is to find out a way to reduce the change of I_(o) due tofluctuation in the input mains voltage.

By the law of conservation of energy, input power is equal to the powerentering the diode bridge, assuming that the input inductor L_(s) hasnegligible resistance. Also and note that V₂₁ and I_(S) are in phase asshown in FIG. 19( b).

V_(S)I_(S) cos φ=V₂₁I_(S)  (4)

where V₂₁ is the fundamental component of V₂.

Similarly, the input power is also equal to the output power of thevalley-fill circuit, assuming that the power loss in the diode rectifierand valley-fill circuit is negligible.

$\begin{matrix}{{V_{S}I_{S}\cos \; \varphi} = {{{\overset{\_}{V}}_{3}{\overset{\_}{I}}_{o}} = {{\frac{3}{4}V_{d\; c}{\overset{\_}{I}}_{o}} = {{{\overset{\_}{I}}_{o}^{2}R} + {{\overset{\_}{I}}_{o}V_{o}}}}}} & (5)\end{matrix}$

If the inductor winding resistance is negligible, R=0, leading to

$\begin{matrix}{V_{o} = {\frac{3}{4}V_{d\; c}}} & (6)\end{matrix}$

Using Fourier analysis on the waveform of V₂, the fundamental componentV₂₁ of V₂ can be determined as:

$\begin{matrix}{V_{21} = {{\frac{\left( {2 + \sqrt{2}} \right)V_{d\; c}}{\pi}{\sin \left( {{\omega \; t} - \varphi} \right)}} = {{1.086 \cdot V_{d\; c}}{\sin \left( {{\omega \; t} - \varphi} \right)}}}} & \left( {7a} \right)\end{matrix}$

The root-mean-square value of V₂₁ is therefore

$\begin{matrix}{{V_{21{\_ rms}}{\frac{1.086}{\sqrt{2}} \cdot V_{d\; c}}} = {0.77 \cdot V_{d\; c}}} & \left( {7b} \right)\end{matrix}$

Dividing (4) by (5) to relate V₂₁ and V_(dc), and using (7b), one canrelate I_(s) and Ī_(o).

0.77V_(dc)I_(S)=0.75V_(dc)Ī_(o)

I_(S)=0.974Ī_(o)  (8)

Now consider the equivalent circuit and the vectorial relationshipbetween V_(s) and V₂₁ as shown in FIG. 21.

From FIG. 21

V _(S) ² =V ₂₁ ²+(ωL _(S) I _(s))²  (9)

and

$\begin{matrix}{{\overset{\rightarrow}{I}}_{s} = \frac{{\overset{\rightarrow}{V}}_{s} - {\overset{\rightarrow}{V}}_{21}}{j\; \omega \; L_{s}}} & (10)\end{matrix}$

From (6), it can be seen that V₂₁ depends on V_(dc), which isapproximately close to 1.33V_(o) (approximated as a constant value).With the help of (8),

$\begin{matrix}{{\overset{\_}{I}}_{o} = \frac{V_{S} - V_{21}}{{0.974 \cdot \omega}\; L_{S}}} & (11)\end{matrix}$

Differentiating (11) will lead to

$\begin{matrix}{{\Delta \; {\overset{\_}{I}}_{o}} = \frac{\Delta \; V_{S}}{{0.974 \cdot \omega}\; L_{S}}} & (12)\end{matrix}$

Equation (12) is the important equation which shows that the inputinductance Ls can be used to reduce the change of average output loadcurrent ΔĪ_(o) for a given change in the input AC mains voltage ΔV_(S).Take an example. For an AC mains of 50 Hz, the angular frequency ω isequal to 100π, that is 314.16. For an Ls of 1H, the effect of inputvoltage fluctuation on the output average current will be reduced by314.16 times as shown in (12). For an Ls of 2H, the reduction will be618 times. For this sensitivity control to be effective, the size of theinput inductor Ls has to be reasonably large (typically near to or inthe order of Henry).

In order to provide a conducting path for the inductor current in Ls incase there is any problem in other part of the circuit which may createa discontinuation of current, a capacitor Cs can be placed to the secondend of the input inductor as shown in FIG. 22. This LsCs arrangementwill also play the additional role of input filter. But the main purposeof using a “large” Ls here is to reduce the sensitivity of the outputload current (and thus output load power) of the proposed circuit toinput voltage fluctuation.

In order to relate Ī_(o) with Vs, we start with modifying (9) with thehelp of (7b) and (8) gives:

V _(S) ²=(0.77V _(dc))² +[ωL _(S)(0.974Ī _(o))]²  (13)

Using (6), (13) becomes:

$\begin{matrix}{V_{S}^{2} = {\left\lbrack {(0.77)\left( \frac{4}{3} \right)V_{o}} \right\rbrack^{2} + \left\lbrack {\omega \; {L_{S}\left( {0.974{\overset{\_}{I}}_{o}} \right)}} \right\rbrack^{2}}} & (14)\end{matrix}$

Solving (14) gives:

$\begin{matrix}{{\overset{\_}{I}}_{o} = \frac{\sqrt{V_{s}^{2} - \left( {1.072 \cdot V_{o}} \right)^{2}}}{{0.974 \cdot \omega}\; L_{s}}} & (15)\end{matrix}$

Note that V_(o) can be determined from the number of LED devices in theLED strings. If Ls is chosen, then (15) provides the relationshipbetween the average output current and the input ac mains voltage.

The LED load power is therefore:

${\overset{\_}{P}}_{o} = {V_{o} \cdot \frac{\sqrt{V_{s}^{2} - \left( {1.072 \cdot V_{o}} \right)^{2}}}{{0.974 \cdot \omega}\; L_{s}}}$

From the above it can be seen that by providing an input inductor inseries between the AC supply voltage and the diode rectifier thesensitivity of the LED power to fluctuation in the AC supply voltage canbe reduced.

Indeed the provision of an input inductor in series between the ACsupply voltage and the diode rectifier may have useful applications as ameans for limiting variations in the power of the LED load in circuitsthat do not include voltage ripple reduction. FIG. 23 shows an exampleof such a circuit where the input inductor L_(s) is provided in seriesbetween the AC supply voltage V_(s) and a diode rectifier the output ofwhich is provided directly to the load. As with the circuit of FIG. 22 acapacitor C_(s) may be provided in parallel between the input inductorand the diode rectifier to provide a conducting path in the event of anyshort-circuit or other problem in another part of the circuit, and alsoto provide a filtering function.

In another embodiment, the lighting system described above can become adimmable system by using a variable input inductor L_(s), as shown inFIG. 24. As explained previously, the use of the input inductor of areasonably large size is to reduce the LED power sensitivity againstinput voltage variation. The relationship of the variation of the outputcurrent (which affects the LED power) with the input voltage variationhas been shown as:

${\Delta \; {\overset{\_}{I}}_{o}} = \frac{\Delta \; V_{S}}{{0.974 \cdot \omega}\; L_{S}}$

The output dc current can be expressed as:

${\overset{\_}{I}}_{o} = \frac{V_{S} - V_{21}}{{0.974 \cdot \omega}\; L_{S}}$

This equation means that the size of the input inductor can affect thepower of the LED load. If the inductance of the input inductor Ls can bechanged, a dimming function becomes possible. By using a variableinductor Ls as shown in FIG. 24, the power control of the LED load canbe achieved.

The variable inductor can be implemented in various forms. For example,FIG. 25 shows an inductor with tapping control. By controlling theswitch or switches, labeled as S1 and S2 in the present embodiment, todetermine the number of turns in the inductor, the inductance value canbe controlled. FIG. 26 shows another implementation of a variableinductor using a DC current in an auxiliary winding to alter themagnetic property (such as saturation level) of the core in order tovary the inductance value.

A further aspect of the invention refers more generally to valley-fillcircuits used in reducing the DC output voltage ripple and/or currentripple in AC-DC power conversion. Based on the ratio of the capacitorsused in the valley-fill circuits, the output voltage ripple can befurther controlled and reduced. Thus, it can be used to provide a DCvoltage source with an even more reduced voltage variation than that,for example, described above. Further, if an inductor is connected tothe output of the valley-fill circuit in order to turn the voltagesource into a current source, a current source with a further reducedcurrent ripple can also be generated.

This further aspect of the present invention is particularly suitable toa variety of applications in which a fairly constant output currentsource is required. Thus, although this aspect of the invention will bedescribed with reference to drivers for LED loads for general lightingapplications such as those described above, this aspect of the inventioncan be applied more generally.

Valley-fill circuits have been proposed as passive methods (withoutactive power switches) for input power factor corrections in AC-DC powerconversion circuits and have been adopted in low-cost applications suchas electronic ballasts and AC-DC converters. Modified versions ofvalley-fill circuits have also been suggested for power factorcorrection. Two common features shared by these valley-fill applicationsare (i) the valley-fill circuits are used primarily for shaping theinput current in the AC-DC power conversion circuit for improving thepower factor and (ii) use capacitors of equal capacitance value in theindividual circuits.

As described above, valley-fill circuits are used for reducing theoutput DC voltage ripple or variation so that a fairly constant currentsource can be generated with the help of a filter inductor. Onepreferred embodiment of such a circuit is shown in FIG. 18. Intraditional applications of valley-fill circuits, the two capacitors C1and C2 are of the same capacitance value. This will allow the outputvoltage to be smaller than that of a diode bridge, i.e. the outputvoltage ripple can be reduced by about 50%.

If the valley-fill circuit in FIG. 18 is not used, the front-end dioderectifier provides a rectified output voltage as shown in FIG. 27. Itcan be seen that the rectified DC voltage peaks at the maximum value ofthe input sinusoidal voltage and then drops to zero. However, when thevalley-fill circuit is provided with C1=C2=220 μF, the output DC voltagefeeding the filter inductor L and LED load has a much reduced voltageripple as displayed in FIG. 28( a). In this case, the maximum voltage isclose to 180V and minimum voltage is close to 90V.

It should be noted that smaller capacitors such as C1=C2=22 μF can alsobe used. For capacitance of this low magnitude, electrolytic capacitorswhich have short lifetimes are not needed. FIG. 28( b) shows the outputDC voltage of the valley-fill circuit with these smaller capacitors.Also voltage charging and discharging in the capacitors becomes obvious,as shown in FIG. 28( b), but the average voltage is close to that inFIG. 28( a).

In the cases of FIG. 28( a) and FIG. 28( b), the output DC voltageripple is about 50% of the maximum value. This is a typical feature ofvalley-fill circuits where C1 and C2 are identical. When the voltage V₂is at its peak, the output voltage of the valley-fill circuit, which isclamped by the voltage across the LED load, will reach its maximumvalue. The rectified input current charges the two identical capacitorsC1 and C2 through the diode D₂ equally and hence the two capacitorvoltages are equal at this moment. Note that this voltage for C2 (halfof the maximum DC voltage) is the maximum voltage of C2. Therefore, onlyfull maximum voltage or half maximum voltage levels appear in the outputvoltage of the valley-fill circuit if C1 and C2 are large enough and ofequal capacitance. Note that the lower DC voltage is actually thevoltage across C2. With a voltage ripple reduced to 50%, the size of thefilter inductor L can be reduced too.

However, the output DC voltage of the valley-fill circuit can be furtherreduced so as to further reduce the output ripple in the DC currentand/or the size of the filter inductor. For capacitors connected inseries, it is well known that the voltage of across each capacitordepends on the size of the capacitance. FIG. 29 shows one example of twocapacitors connected in series. Note that the current flow into thisseries circuit branch is the same in the two capacitors regardless oftheir capacitance. That is to say, the capacitors have the same amountof charge for a given series current flow. The voltage across eachcapacitor is inversely proportional to the size of the capacitor.

In order to increase the lower DC voltage level (i.e. voltage acrossC2), one can select the capacitance of C2 to be smaller than that of C1(i.e. C1>C2). This rule ensures that the voltage across C2 is higherthan 50% of the maximum DC voltage. In order to confirm this concept, C1and C2 are changed to 6600 μF and 330 μF, respectively. FIG. 30 showsthe resulting output DC voltage of the valley-fill circuit if this isdone. It can be seen that the voltage ripple is now reduced to about30%.

Thus, specifying C1>C2 further reduces the output voltage ripple in thevalley-fill circuit so as to reduce the ripple in the output inductorcurrent and/or the size of the filter inductor.

It will be noted that any capacitors, including electrolytic capacitors,can be used. However, non-electrolytic capacitors are preferred sincethese lead to longer lifetimes and higher reliability.

Although the invention has been described with reference to specificexamples, it will be appreciated by those skilled in the art that theinvention can be embodied in many other forms. It will also beappreciated by those skilled in the art that the features of the variousexamples described can be combined in other combinations.

1. An LED lighting system comprising in sequence: a rectificationcircuit for rectifying an AC input power and generating a rectified DCpower; a first circuit for reducing the voltage ripple of said rectifiedDC power; a second circuit for generating a current source; and at leastone LED receiving said current source as an input.
 2. The LED lightingsystem of claim 1, wherein said voltage ripple reducing circuit is avalley-fill circuit located between said rectification circuit and saidsecond circuit.
 3. The LED lighting system of claim 2, wherein saidvalley-fill circuit includes a first capacitor and a second capacitor.4. The LED lighting system of claim 3, wherein the first and secondcapacitors have the same capacitance.
 5. The LED lighting system ofclaim 3, wherein the first and second capacitors have differentcapacitances such that the voltage ripple of said rectified DC power isfurther reduced.
 6. The LED lighting system of claim 2, wherein saidvalley-fill circuit includes a voltage-doubler.
 7. The LED lightingsystem of claim 1, wherein said second circuit comprises an inductor. 8.The LED lighting system of claim 1, wherein said second circuit is acurrent ripple reduction circuit.
 9. The LED lighting system of claim 8,wherein said current ripple reduction circuit comprises a coupledinductor with a capacitor.
 10. The LED lighting system of claim 1,wherein means are provided for controlling the sensitivity of the LEDpower to fluctuations in the voltage of the AC input power.
 11. The LEDlighting system of claim 10, wherein said means for controlling thesensitivity of the LED power to fluctuations in the voltage of the ACinput power comprises an inductor provided in series between the ACinput and the diode rectification circuit.
 12. The LED lighting systemof claim 11 further comprising a capacitor provided in parallel betweenthe said inductor and the said diode rectification circuit.
 13. The LEDlighting system of claim 11, wherein said inductor is a variableinductor controllable such that the at least one LED is dimmable. 14.The LED lighting system of claim 1, wherein means are provided forvarying the AC input power such that the at least one LED is dimmable.15. The LED lighting system of claim 14, wherein said means for varyingthe AC input power comprises a variable inductor provided in seriesbetween the AC input and the diode rectification circuit.
 16. The LEDlighting system of claim 15, wherein said variable inductor is providedwith tapping control.
 17. The LED lighting system of claim 15, whereinsaid variable inductor is provided with an auxiliary winding having a DCcurrent to alter a magnetic property of the core in order to vary theinductance value of said variable inductor.
 18. The LED lighting systemof claim 1, wherein the power supplied to said at least one LED ispermitted to vary, and wherein the operating and/or design parameters ofsaid at least one LED are chosen such that the variation in luminousflux resulting from the variation in power is not observable to thehuman eye.
 19. An LED lighting system comprising: an AC input powersource; a rectification circuit for rectifying an AC input power andgenerating a rectified DC power; and an inductor provided in seriesbetween the AC input power source and the rectification circuit.
 20. TheLED lighting system of claim 19 further comprising a capacitor providedin parallel between the said inductor and the said diode rectificationcircuit.
 21. The LED lighting system of claim 19, wherein said inductoris a variable inductor controllable such that the LED lighting system isdimmable.
 22. The LED lighting system of claim 21, wherein said variableinductor is provided with tapping control.
 23. The LED lighting systemof claim 21, wherein said variable inductor is provided with anauxiliary winding having a DC current to alter a magnetic property ofthe core in order to vary the inductance value of said variableinductor.
 24. A method of operating a LED lighting system comprising thesteps of: rectifying an AC input voltage to generate a rectified DCpower; reducing the voltage ripple of said rectified DC power;generating a current source from said voltage ripple reduced rectifiedDC power; and providing said current source as an input to at least oneLED, wherein the power supplied to said at least one LED is permitted tovary, and wherein the operating and/or design parameters of said atleast one LED are chosen such that the variation in luminous fluxresulting from the variation in power is not observable to the humaneye.
 25. The method of claim 24, wherein a thermal characteristic of theat least one said LED is chosen such that the variation in luminous fluxresulting from the variation in power is not observable to the humaneye.
 26. The method of claim 25, wherein said thermal characteristiccomprises the design of the heatsink.
 27. The method of claim 25,wherein said thermal characteristic comprises the provision of forcedcooling or natural cooling.
 28. The method of claim 24, wherein avalley-fill circuit is used to reduce the voltage ripple of therectified DC power.
 29. The method of claim 28, wherein the valley-fillcircuit is provided with a first capacitor and a second capacitor. 30.The method of claim 29, wherein the first capacitor and the secondcapacitor have the same capacitance.
 31. The method of claim 29, whereinthe first capacitor is selected with a different capacitance to thesecond capacitor such that the valley-fill circuit is used to furtherreduce the voltage ripple of the rectified DC power.
 32. The method ofclaim 28, wherein said valley-fill circuit includes a voltage-doubler.33. The method of claim 24, further comprising the step of reducing thecurrent ripple of said current source.
 34. The method of claim 33,wherein a coupled inductor with a capacitor is used to reduce thecurrent ripple.
 35. The method of claim 24, further comprising reducingthe sensitivity of the LED power to fluctuations in the AC inputvoltage.
 36. The method of claim 34, further comprising providing aninductor to reduce the sensitivity of the LED power to fluctuations inthe AC input voltage before rectifying the AC input voltage.
 37. Themethod of claim 36, further comprising controllably varying the inductorsuch that the at least one LED is dimmable.
 38. The method of claim 24,further comprising varying the AC input voltage such that the at leastone LED is dimmable.
 39. The method of claim 38, further comprisingproviding a variable inductor to vary the AC input voltage.
 40. Themethod of claim 39, further comprising controlling the variable inductorwith tapping control.
 41. The method of claim 39, further comprisingproviding the variable inductor with an auxiliary winding having a DCcurrent and altering a magnetic property of the core in order to varythe inductance value of said variable inductor.
 42. A system including avalley-fill circuit for generating a DC output voltage, said circuitincluding a first capacitor and a second capacitor, wherein the firstand second capacitors have different capacitances such that the voltageripple of the DC output voltage is reduced.
 43. A system as claimed inclaim 42 further comprising a current source circuit to receive andconvert said voltage ripple reduced DC output voltage into a currentsource, said current source thereby having a reduced current ripple. 44.The system of claim 43, wherein the current source circuit includes aninductor.
 45. The system of claim 43, connected as an input to a loadrequiring a relatively constant current source.
 46. The system of claim45, wherein said load is an LED lighting system in accordance withclaim
 1. 47. A method of generating a DC output by using a valley-fillcircuit including a first capacitor and a second capacitor, wherein thefirst and second capacitors have different capacitances such that a DCoutput voltage with reduced voltage ripple is generated.
 48. The methodof claim 47, wherein the DC output voltage is converted to a currentsource having a reduced current ripple.
 49. The method of claim 48,wherein an inductor is used to convert the DC output voltage to acurrent source.
 50. The method of claim 48, further comprisingconnecting the current source as an input to a load requiring arelatively constant current source.
 51. The method of claim 50, whereinsaid load is an LED lighting system in accordance with claim 1.